1. Field of the Invention
The invention concerns an intruder detection system, particularly an improved system for detecting and signaling the presence of a human being within one or more defined areas.
2. Prior Art
Numerous systems have been designed and are presently in use to sense and signal the presence of an intruder within a defined area. Some such systems are active in nature, producing a signal within or to define the secure area, the signal being affected by the presence of an intruder and this affect being employed to actuate an alarm or other signal. Not only do such active systems require a continuous energy input, but their presence also can be detected by the intruder, permitting them to be avoided or otherwise negated. Other intrusion detection system are of a passive nature. Most passive systems detect the presence of an intruder in a defined area by sensing and responding to visible or invisible radiation of the intruder. Rossin U.S. Pat. No. 3,839,640, dated Oct. 1, 1974, presents an example of an intruder detection sensor responsive to infrared radiation.
Intrusion detection systems are subjected to a number of non-intruder related inputs, all of which must be distinguished from an intruder related input if the system is to prove practical in use. Inputs which can cause false alarms included reflected sunlight such as from an airplane or automobile or light being directed onto the detector from automobile headlights or a flashlight out of the detection area. An intermittent electric heater, or a flickering light bulb, or fluttering foliage or drape in the detector's field of view also can actuate present systems, causing a false alarm.
An intrusion detection system should avoid false alarms from such environmental occurrences. Of course, it must detect the presence of an intruder, even when the intruder is taking steps to avoid detection, such as wearing white clothing while crossing a white background or while crossing a monitored background only after having adjusted his external temperature to closely approximate that of the background temperature. In short, detecting the presence of an intruder while not producing numerous false alarms presents a significant problem, one which has only been partially solved by previous intruder detection systems.
Many intruder detection systems are battery-powered. Often, such systems employ a circuit to detect battery voltage and to produce an alarm when that voltage drops below a certain level. This, too, results in a false alarm, since the alarm does not signal the presence of an intruder, but rather a drop in battery voltage below a given level.